1. Field of the Invention
The present invention relates to temperature control of a device whose characteristics depend on temperature, and more particularly to the constant-temperature control by detecting the device temperature by means of a temperature sensor provided near the device.
2. Description of the Prior Art
Generally, devices such as a semiconductor device change in characteristic depending on temperature. Therefore, constant-temperature control is indispensable for operating such a device stably.
Japanese Patent Laid-open Publication No. HEI 1-243488, for example, discloses a module having a semiconductor laser incorporated therein. Since the semiconductor laser has such a property that its oscillating wavelength shifts depending on temperature, the temperature of the semiconductor laser is kept constant by using a thermistor for temperature detection and a Peltier cooler for temperature adjustment both provided near the semiconductor laser. The constant-temperature control is achieved by the Peltier cooler generating or absorbing heat based on the detected temperature.
However, since a temperature difference exists between the positions of installation of the semiconductor laser and the thermistor, a temperature detected by the thermistor really contains errors therein and does not indicate an actual temperature of the semiconductor laser. Generally, when temperature detection is made by mounting a temperature sensor near a device, it is difficult to eliminate this temperature difference itself drastically. Moreover, this temperature difference results from a difference between the actual temperature and the surroundings temperature of the device and therefore it cannot be discriminated from the detected temperature. This deteriorates accuracy in the temperature control of this kind and becomes a primary factor in a device characteristic shift such as the temperature shift of an oscillating wavelength.
To suppress such a device characteristic shift, there has so far been adopted a construction in which a hold member for supporting the semiconductor laser is made of a high heat conducting material and a thermistor is attached at a position near the semiconductor laser so that the above-described temperature difference can be reduced to a practically negligible degree (see above-described Japanese Patent Laid-open Publication No. HEI 1-243488).
However, if operating surroundings temperature becomes higher or lower than a normal operating range, a difference between the operating surroundings temperature and the control target temperature of the semiconductor laser will be much greater than estimated, so a temperature difference between the detected temperature and the actual temperature of the device will become a very important problem.
Also, if a temperature sensor is mounted at a position adjacent to the semiconductor laser to suppress such a temperature difference, there will occur the need for leaving a space for the temperature sensor near the semiconductor device, and the degree of freedom on the module design will be restricted.